I've noticed that when I'm running at a continuous strong pace, like in a tempo run on the track, I can sometimes hear my footfalls in my breath. It turns out I can control the magnitude of this effect by either tightening or loosening my core. Loosening my core increases the raspy sound on foot-strike whereas it's not difficult to tighten up enough that the sound is not noticeable.
Two questions: 1) has anyone else observed this and 2) if so, which side of this level of core tension is desirable?